1. Field of the Invention
The invention relates to tubular or sleeve-shaped heating and measurement devices, especially glow plugs, heating plugs for preheating water, measurement and sensor elements.
2. Description of Related Art
It is desirable to produce glow plugs, heating plugs for preheating water, measurement and sensor elements to be as slender as possible for reasons of minimization of material consumption, the amount of space required, and weight. However, thermal stresses occur during the operation of the more slender and narrowly-dimensioned glow plugs known in the prior art lead to adverse effects on the function of these prior art glow plugs. In conventional glow plugs produced with an annular gap, vapor bubble formation and coking occurs in the annular gap thereby resulting in faulty measurements from a correspondingly built sensor.
Due to the reduction in the diameter of these devices though slenderizing together with the simultaneous demand for longer dimensioned rod glow plugs, major technical difficulties arise because of the required tolerances and the necessity of economical production of these devices. In particular, major technical problems in the manufacturing of such devices include adherence to concentricity tolerances, difficulties in sealing between the glow plug body and the glow tube and space problems in the area of the control spiral in rod glow plugs.
As an example, the prior art glow plug 100 as shown in FIG. 1 consists of the glow plug body 15, into which glow plug tip tube 16 is installed. The glow tube 16 includes a heating element 4 and a control element 5 that are embedded in an electrically insulating material for conducting heat, such as MgO. In the terminal-side area in the body 15, there is provided a connection lead 10. This area of the glow plug 100 is sealed by a seal 2 relative to the glow plug tip tube 16. The terminal-side area includes a cavity around the connection lead 10 which connects to an electric terminal 6. This cavity is sealed by another seal 3. This terminal-side of the glow plug body 15 has a thread 7 and a polyhedron 8. An annular gap 14 is provided proximate to the glow plug tip tube 16 in the combustion space-side exit area between the glow plug tip tube 16 and the glow plug body 15. As can be seen, the annular gap 14 is formed by increasing the diameter of the hole in the glow plug body 15. As can be readily appreciated, such prior art glow plugs are relatively complicated with many parts which are difficult to manufacture and assemble.